1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a field sequential driving method and a liquid crystal display device using the same.
2. Description of the Related Art
Recently, personal computers and televisions have become lightweight and flat, and accordingly display devices are being required to be more light weighted and thinner. Thus, for use instead of a cathode ray tube (CRT), flat panel displays including a liquid crystal display (LCD) have been developed.
An LCD device utilizes two substrates and a liquid crystal material having an anisotropic dielectric constant injected therebetween, in which an electric field is applied to the liquid crystal material. The amount of light from an external light source transmitted through the substrates is controlled by intensity of the electric field to obtain a desired image signal.
Such an LCD is the most common type of the flat panel displays, and especially, a thin film transistor (TFT)-LCD using a TFT as a switching element is most commonly used.
Each pixel in the TFT-LCD can be modeled using a capacitor having a liquid crystal as a dielectric material, that is a liquid crystal capacitor. An equivalent circuit diagram of such a pixel is shown in FIG. 1.
As shown in FIG. 1, each pixel in an LCD device includes a TFT 10 having a source electrode and a gate electrode respectively coupled to a data line Dm and a scan line Sn, a liquid capacitor Cl coupled between a drain electrode of the TFT 10 and a common voltage source Vcom, and a storage capacitor Cst coupled to the drain electrode of the TFT 10.
As can be seen in FIG. 1, the TFT 10 is turned on when a scan signal is applied to the scan line, and a data voltage Vd supplied to the data line Dm is applied to each pixel (not shown) through the TFT 10. Then, an electric field corresponding to a difference between a pixel voltage Vp and the common voltage Vcom is applied to a liquid crystal (equivalently shown as a liquid crystal capacitor Cl in FIG. 1), and light transmittance is determined by intensity of the electric field. Here, the pixel voltage Vp is maintained for one frame scan or one field, and the storage capacitor Cst is auxiliarily used to maintain the pixel voltage Vp applied to the pixel electrode.
In general, methods of displaying a color image on an LCD device can be classified into a color filter method and a field sequential driving method.
An LCD device employing the color filter method forms a color filter layer having 3 primary colors (red, green, and blue) on one of substrates, and controls the amount light transmitted to the color filter to express a desired color. An LCD employing the color filter method adjusts the amount of light from a single light source transmitted through red, green, and blue color filters, and combines the red, green, and blue lights to display a desired color.
Such an LCD device displaying colors using a single-light source and three color filter layer requires three times or more pixels, compared to displaying monochrome, to respectively correspond to red, green, and blue color areas. Accordingly, a sophisticated manufacturing technology is required to obtain a high resolution image.
Moreover, adding a separate color filter layer on the substrate of the LCD causes the manufacturing of the LCD to be complicated, and light transmittance of the color filter must be considered as well.
On the other hand, an LCD employing the field sequential driving method periodically and sequentially turns on/off independent red, green, and blue signals, and synchronously applies a corresponding color signal to the pixel in accordance with the turn on/off period to thereby obtain a full-colored image. In other words, the field sequential driving method uses persistence of vision to display a colored image by way of outputting the red, green, and blue (RGB) lights from RGB light sources (i.e., backlights) and time-dividing the red, green, and blue lights, and sequentially displaying the time-divided red, green, and blue lights on a pixel instead of dividing the pixel into three pixels for red, green, and blue colors.
The field sequential driving method can be classified into an analog driving method or a digital driving method.
The analog driving method predetermines a plurality of gradation voltages corresponding to a total number of gradations to be displayed, and selects a gradation voltage corresponding to gradation data from the plurality of gradation voltages to drive a liquid crystal panel to thereby express gradation using the amount of light transmitted corresponding to the gradation voltage applied to the liquid crystal panel.
FIG. 2 illustrates a driving voltage and the amount of transmitted light of an LCD panel employing a conventional analog driving method. As shown therein, the driving voltage represents a voltage applied to the liquid crystal, and the optical transmittance represents a ratio of the amount of light transmitted through the liquid crystal to the amount of incident light. In other words, the optical transmittance represents the degree of distortion of the liquid crystal so that the light can pass therethrough.
Referring to FIG. 2, a driving voltage at V11 level is applied to the liquid crystal in an R-field period Tr for displaying a red color and the amount of light transmitted through the liquid crystal corresponds to the driving voltage. In a G-field period Tg for displaying a green color, a driving voltage at V12-level is applied and a corresponding amount of light is transmitted through the liquid crystal. Further, in a B-field period Tb for displaying a blue color, a driving voltage at V13 level is applied and a corresponding amount of light is transmitted through the liquid crystal. By adding the red, green, and blue lights respectively transmitted through the Tr, Tg, and Tb, a desired colored image can be displayed.
On the other hand, the digital driving method regulates driving voltages applied to the liquid crystal and controls a voltage application time to thereby express gradations (i.e., grayscales). According to the digital driving method, the gradations are expressed by maintaining the regulated driving voltage and adjusting a timing or duration of the voltage application to control an accumulated amount of light transmitted through the liquid crystal.
FIG. 3 illustrates waveforms that explain a driving method of an LCD device employing a conventional digital driving method. Waveforms of a driving voltage in accordance with a predetermined number of bits of driving data and corresponding optical transmittance of a liquid crystal are illustrated.
As shown in FIG. 3, a 7-bit digital signal is provided as gradation waveform data for each gradation, and a corresponding gradation waveform is applied to the liquid crystal. The optical transmittance of the liquid crystal is determined according to the applied gradation waveform, thereby expressing the gradations.
The LCD device employing the conventional field sequential method uses a light emitting diode (LED) as the backlight of R, G, and B, and sequentially drives a red LED, a green LED, and a blue LED. In other words, the field sequential method has an R-field period for red color, a G-field period for green color, and a B-field period for blue color, and the red, green, and blue LEDs are sequentially turned on to emit red, green, and blue lights. Each of red, green, and blue data is applied to the liquid crystal and accumulated in the respective field periods, and a colored image can be displayed through the accumulated red, green, and blue lights.
FIG. 4 shows a relationship between each of conventional LEDs that respectively emit red, green, and blue lights, and a light source controller driving the conventional LEDs.
As shown in FIG. 4, the conventional LEDs include a red LED (RLED), a green LED (GLED), and a blue LED (BLED), and these LEDs are coupled to the light source controller. When gradation data is applied to a pixel, the light source controller immediately turns on the RLED, GLED, and BLED in sequence, and applies a forward voltage Vf to the respective LEDs to thereby emit light providing sufficient luminance. In FIG. 3, anodes of the RLED, GLED, and BLED are coupled to a common terminal VLED supplying the forward voltage, and cathodes of the RLED, GLED, and BLED are respectively coupled to selection terminals R_OUT, G_OUT, and B_OUT. Here, each of the selection terminals R_OUT, G_OUT, and B_OUT is sequentially turned on, and at the same time, the forward voltage is sequentially applied to the RLED, GLED, and BLED to thereby turn them on.
Here, each of the LEDs, namely, RLED, GLED, and BLED, requires different voltage level to be turned on, and different forward voltages Vf result in different forward currents If. Further, the amount of luminance of the red LED RLED, green LED GLED, and blue LED BLED are respectively different according to the forward current If. Here, the forward voltage Vf represents a voltage applied to the LEDs after the LEDs are turned on, and the forward current If represents a current flowing to the LEDs when the forward voltage Vf is applied thereto.
FIGS. 5A and 5B illustrate a relationship between a forward voltage Vf and a forward current If in typical red, green, and blue LEDs, and relative luminance corresponding thereof. FIG. 5A shows a relationship between the forward voltage and a corresponding forward current, and FIG. 5B shows the forward current and corresponding relative luminance or luminance.
As shown in FIG. 5B, relative luminance of the red, green, and blue LEDs are substantially the same when the forward current If applied thereto is set to be 20 mA. For white balancing, the green LED and the blue LED respectively require 3.4V and 3.25V of forward voltages but the red LED requires only 2.1V of forward voltage which is relatively lower than the forward voltages of the green and blue LEDs. The forward voltage is supplied from a terminal VLED of the light source controller, and the light source controller respectively applies associated forward voltages to the red, green, and blue LEDs in sequence. Here, the forward voltage 3.4V of the green LED and the forward voltage 3.25V of the blue LED have a similar voltage value, but the forward voltage 2.1V of the red LED is comparatively lower than the forward voltages of the green and blue LEDs, thereby generating a voltage ripple in the light source controller. In other words, variation of forward voltages produces the voltage ripple, thereby resulting in many problems in controlling the amount of light emitted from the respective LEDs.